US3398879A - Asymmetric ion pump and method - Google Patents

Description

Aug. 27, 1968 B. D. JAMES ET AL 3,398,879

ASYMMETRIC ION PUMP AND METHOD Filed Oct. 7, 1966 2 Sheets-Sheet 1 W \22 i/22 I9 (22 INVENTORS BRIAN DAVID JAMES THEODORE K. TOM

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ATTORNEYS Aug. 27, 1968 B. D. JAMES E ASYMMETRIC ION PUMP AND METHOD 2 Sheets-Sheet 2 Filed Oct.

CONVENTIONAL DIODE P(X 10- TORR) nnunnu ANODE F/G f0 COLLECTOR ENVELOP ODE 1 --D!FFERENT|AL SPUTTER ION PUMP-- 1O 2O 3O 4O 5O 6O 7O 8O HMIN.)

48 CATHODE 2 F /G. -TR|oDE- D i P P(X 10- TORR) l 30 HMIN.)

E l nw INVENTORS BRIAN DAVID JAMES THEODORE K. TOM

HMIN.)

ATTORNEYS United States Patent lice 3,398,879 ASYMMETRIC ION PUMP AND METHOD Brian David James, Menlo Park, and Theodore K. Tom,

Sunnyvale, Calif., assignors, by mesne assignments, to

The Perkin-Elmer Corporation, Norwalk, Conn., a corporation of New York Filed Oct. 7, 1966, Ser. No. 600,297 3 Claims. (Cl. 230-69) This application is a continuation-in-part of copending application Ser. No. 511,516 entitled, Asymmetric Ion Pump and Method and filed Dec. 3, 1965.

This invention relates generally to a vacuum pump of the electronic type and more particularly to so-called ion pumps, and to a method of ion pumping.

Electronic pumps employing cold cathode discharge in magnetic fields are well known in the art. In such pumps, an electric field is provided between a cathode and anode placed in an uni-directional magnetic field. Electrons travelling from the cathode to the anode are deflected by the magnetic field and traverse a relatively long path. These electrons collide with the molecules of the gases to be pumped and form ions, atoms (disassociated molecules) and metastable molecules. The latter two species are captured chiefly at the surface of the anode by the gettering action of material which is sputtered from the cathode by ion bombardment.

Prior art ion pumps for pumping inert gases, such as argon and helium, included slotted cathode pumps, commonly called air-stable pumps; triode ion pumps; as well as diode ion pumps. Ion pumps, existing at this time, exhibit instability in the pumping of inert gases. This instability arises when the inert gas molecules, that have been pumped by burial in the cathodes of the pump, are re-emitted causing a sudden rise in pressure. These pressure bursts or instabilities cause considerable difficulty in vacuum systems and sometimes cause a vacuum system to stall and lose its vacuum altogether.

It is a general object of the present invention to provide an ion pump capable of effectively pumping inert and active gases stably, and a method of ion pumping.

It is another object of the present invention to provide an ion pump which has a relatively high pumping speed for inert gases.

It is another obect of the present invention to provide an ion pump suitable for pumping inert and active gases which is simple in construction and inexpensive to manufacture.

It is still a further object of the present invention to provide an ion pump which can operate at higher pressures of inert gases without stalling.

It is still another object of the present invention to pro vide an ion pump which can maintain a high vacuum for long periods in the presence of small air leaks which contain inert gases.

The foregoing and other advantages of the present invention will be more clearly apparent from the following description when taken in conjunction with the accompanying drawing.

Referring to the drawing:

FIGURE 1 is a side elevational view, partly in section, of an ion pump incorporating the present invention;

FIGURE 2 is a top view, partly in section, of the ion pump shown in FIGURE 1;

FIGURE 3 is a top view of the anode-cathode assembly of the pump shown in FIGURES 1 and 2;

FIGURE 4 is a side elevational view of the anodecathode assembly shown in FIGURE 3;

FIGURE 5 is a schematic representation of the anodecathode assembly;

FIGURE 6 is a plan view showing a cathode plate;

3,398,879 Patented Aug. 27, 1968 FIGURE 7 is a graph showing a comparison of pumping speeds of a conventional diode ion pump and an ion pump in accordance with the present invention for helium;

FIGURE 8 is a graph showing a comparison of pumping speeds of a conventional diode ion pump and an ion pump in accordance with the present invention for argon;

FIGURE 9 is a graph showing a comparison of pumping speeds of a triode ion pump and an ion pump in accordance with the present invention for argon;

FIGURE 10 is a schematic representation of another embodiment of the invention; and

FIGURE 11 is a schematic representation of a triode pump inconporating the present invention.

FIGURES 1 and 2. illustrate a typical ion pump incorporating the present invention. The ion pump includes a box-like pump chamber 11 supported by legs 12. The chamber includes bottom wall 15, spaced side walls 13 and top wall 14 which carries an inlet conduit 16- provided with flange 17 to connect the ion pump to associated equipment.

A pair of pockets 18- are formed by channels 19 carried on opposed walls 13a and 13b. These pockets communicate with the pump chamber and are each adapted to receive an anodecathode assembly 21 to be presently described.

In the embodiment shown, bar-like permanent magnets 22, which may be ferrite bars, are disposed outside of the pump chamber on each side of the channels. The magnets provide a uni-directional magnetic field across the pockets. Three magnets 22 are disposed on each side of the pump chamber to provide a magnetic field to all of the pockets 1%. Pole pieces 23 provide a shunt across the other walls of the pump chamber to complete the magnetic path so that the reluctance of the path is relatively low. Thus, relatively high magnetic fields are present at the gaps which include the pockets 18.

A- typical anode-cathode assembly 21 is more clearly shown in FIGURES 3 and 4. The assembly includes a cellular anode 26. The cells may have any convenient cross-section formed by cooperating cell walls. The anode shown is formed by joining together a plurality of cylindrical anode members 27. This forms a cellular anode structure with the axis of the cylindrical members 27 disposed substantially perpendicular to the plane of the assembly. The anode 26 is supported between and spaced from a pair of cathode plates or members 28. The support includes brackets 31 secured to the sides of the anode assembly which engage brackets 32 carried by the cathode plates. The engagement between the brackets includes a shield 33 and a coaxial cylindrical insulator 34. The insulator and shield are secured to the bracket 32 by a screw 36. The other end of the insulator 34 receives screw 37 which engages the anode bracket 31. End brackets 40 serve to maintain the cathode plates in spaced relationship. The assembly is supported from the walls of the pump chamber by brackets 39 and electrical connection is made to the anode through tab 41.

As previously described, one such anode-cathode assembly is inserted in each of the pockets 19 and is suitably supported from the walls of the pump chamber by the brackets 39.

Referring to FIGURE 1, electrical connection is made to the anode through the wall of the chamber. The connection includes a strap 42 suitably connected to the tab 41 as, for example, by a screw. Connection is also made to the anode of an adjacent assembly 21 by a strap 43 interconnected between the tabs 41 of the assemblies. A conventional lead-through including stand-01f insulator and connector 44 is provided for making electrical connection to strap 42 through the walls of the chamber.

In prior art ion pumps, the anode-cathode assembly is symmetric, that is, the electric fields between the anode and adjacent cathodes have equal values, and the reactive material forming the cathode members is of the same reactive material, generally titanium so that the sputtering rates from the two cathodes are substantially equal.

The anode-cathode assembly has a magnetic field H, FIGURE 5, applied thereto in a direction substantially parallel to the walls of the cellular anode 26 and perpendicular to the cathode plates 28. A voltage +V is applied between the anode and the adjacent cathodes to set up electric fields therebetween. Gas atoms within the chamber are ionized by collision with electrons travelling be tween the anode and cathode. The electrons travel in spiral paths because of the magnetic field, and the likelihood of striking a gas molecule is increased. The gas molecules are ionized when they collide with the electrons. The positive ions are accelerated towards the adjacent cathode plate 28 by the electric fields.

As explained above, in the usual configuration, cathodes are made of titanium so that if the ion is of a chemically active gas such as nitrogen, it has a good chance of combining with the titanium to yield a stable solid compound titanium nitride, inwhich case it is permanently removed at the cathode. The ion also serves to sputter off a number of titanium atoms from the surface. The titanium atoms travel to and are deposited onto the walls of the cellular anode 26. The sputtered atoms serve to capture or getter the atoms and metastable molecules of the active gases. This gettering is generally done by so-called active gettering, that is, they form compounds with these gas molecules. The foregoing describes essentially the operation of the pump inpumping active gases.

On the other hand, if the ion which is produced by the collisions with the electrons is of a chemically inert gas such as helium or argon, it does not combine with the titanium when it strikes the same though it may cause sputtering. Instead, it buries itself in the titanium cathode due to the large energy it has acquired by being accelerated by the electric field towards the cathode. The removal of the inert gas ion from the gas phase, is however, not necessarily permanent. As pumping proceeds, the surface of the titanium cathode is eroded due to sputtering, or to combination with active gas ions. This, in time, leads to the exposure of the buried inert gas atoms. They are then reemitted into the gas phase.

The electric fields set up between the anode and cathode dictate that the greatest amount of sputtering takes place under the center of the cells and the least amount of sputtering takes place directly under the walls of the anode cells. Referring particular to FIGURE 6, there is shown schematically the appearance of a cathode plate after the pump has been in use. The shaded areas indicate generally those areas of the cathode plate in which the least amount of sputtering has taken place. Thus, it is seen that inert gas atoms which bury themselves in the area under the center of the anode are almost certain to be subsequently re-emitted. Those that eventually bury themselves in the area beneath the anode walls are less likely to be reemitted.

In accordance with the present invention, the sputtering rate of the two cathodes is different whereby there is a net transfer of metal from one cathode to another. Different sputtering rates can be achieved by making the cathodes with metals having different sputtering yields; that is, the number of metal atoms sputtered out for each incident ion is different for the two materials. For example, one cathode plate may be copper and the other titanium. The pump would operate as follows: The inert gas ions accelerated towards the cathodes would collide with both cathodes, but due to the high sputtering yield of the copper compared to the titanium, between three and six times as many copper atoms are sputtered as are titanium atoms. Some of the sputtered copper atoms are deposited on the anode; others return to the cathode of origin; and others are transferred to the opposite (titanium) cathode. Be-

cause of the difference in sputtering yield, there is a net transfer of atoms from the copper cathode to the titanium cathode. These enable inert gas to be pumped by direct occlusion and also by reducing the sputter-induced reemission of previously buried inert gases. Because of the build-up of copper on the opposite cathode, less atoms are sputtered than are deposited at certain areas and the net effect is to reduce re-emission. Since these buried inert gas atoms are not re-emitted, the pump retains a steady positive pumping speed.

Referring more specifically to FIGURE 6, the copper will tend to collect on the titanium cathode in the shaded area 52, thereby building up 'a thick deposit. There is still a sputtering of active titanium metal in the central regions where many ions are incident. These metal atoms serve to pump the active gases.

While in the foregoing example a structure having one titanium and one copper cathode is described, any two metals or alloys having different sputtering yields may be used for inert gas pumping. The preferred ion pump is one in which the cathodes are made of chemically active material having different sputtering yields. The chemically active material gives high pumping efficiency for active gases such as nitrogen, oxygen, and other components of air and yet pumps inert gas ions efficiently. Preferably, at least one cathode should be reactive.

Examples of reactive metals are vanadium, zirconium, molybdenum, tungsten, thorium, hafnium, neodymium, tantalum and their alloys, in addition to titanium and its alloys. Generally, these reactive metals and their alloys have a lower sputtering yield than some non-reactive metals and their alloys. However, they have different sputter yields and thus any two will give cathodes with different sputter yield rates. Anion pump employing cathodes of titanium and tantalum has been found to be highly satisfactory. Non-reactive low sputter yield metals are aluminum and nickel and their alloys. Metals having high sputter yields are zinc, cadmium, gold and silver, and their alloys, in addition to copper and its alloys.

A pump assembly of the type shown in FIGURES 1-4 was constructed. The pump included in alternate pockets anode-cathode assemblies of conventional design including titanium cathodes, and anode-cathode assemblies in accordance with the invention including one titanium and one copper cathode. The pump was operated with helium and argon leaks to test the pumping efficiency for inert gases. The pump was first operated by energizing only the anode-cathode assemblies of conventional design for a period of time, and then operated by energizing only the anode-cathode assemblies in ac cordance with the invention for a period of time. Referring to FIGURE 7, the first portion of the curve shows the pressures obtained during the first period of time in the presence of a helium leak. It is to be noted that pressure bursts periodically occur. The dotted line shows a transfer to pumping during the second period of time in the presence of the same helium leak. It is to be observed that the pumping is continuous and no pressure bursts occur. FIGURE 8 illustrates the results of the same test with an argon leak. FIGURE 9 illustrates the results on argon when comparing a triode ion pump to the diode structure in accordance with this invention. The first portion of the curve shows the instability of the prior art pumps, while the last portion shows pumping by a pump in accordance with this invention.

Asymmetrical sputter rates can also be achieved by employing the same type of cathode surfaces but varying the spacing between the cathode and anodes whereby a higher electric field is present in one region than in the other whereby there is a higher sputter yield in one cathode than the other. The asymmetrical characteristics may be obtained by applying different voltages between the anode and each of the cathode elements, again providing different electric fields and different sputtering rates. The asymmetrical characteristics can also be obtained by providing cathodes of diiferent geometry; for example, one cathode which is perforated to have a lower sputtering yield than the other, or one positioned to have a dilferent sputtering yield than the other. A triode pump can also be constructed in accordance with the invention by providing cathodes having different sputtering yields in the manner described above. The triode pump includes a grounded cellular anode 46, perforated cathodes 47 having a positive voltage applied thereto and grounded collectors 48. The cathodes are constructed to have difierent sputtering yields.

Referring particularly to FIGURE 10, there is schematically shown an anode-cathode assembly in which the lower cathode is grounded, a voltage +V is applied between the lower cathode and the anode, and a voltage +V (where +V is less than V is applied to the upper cathode, thereby providing a different voltage between the anode and the upper and lower cathodes to provide diiferent sputtering rates and pumping in accordance with the invention.

Thus, it is seen that there has been provided a pump which is capable of pumping active as well as inert gases while still maintaining a relatively simple and inexpensive construction.

We claim:

1. In an electronic vacuum pump, means providing a pump chamber, an anode, two cathodes each including reactive material associated with opposite sides of said anode, said anode and cathodes disposed in said chamher, means for applying a voltage between said cathodes and said anode, and means for producing a sputtering yield from one cathode which is different than the vsputtering yield from the other cathode.

2. In an electronic vacuum pump, means providing a pump chamber, an anode, at least two cathodes, said cathodes being of different materials selected from the group consisting of titanium, vanadium, zirconium, molybdenum, tungsten, thorium, hafnium, neodymium, tantalum and their alloys, said anode and cathodes disposed in said chamber, and means for applying a voltage between said cathodes and said anode.

3. In an electronic vacuum pump, means providing a pump chamber, an anode, at least two cathodes associated with said anode, one of said cathodes including titanium and the other of said cathodes including tantalum, said anode and cathodes being disposed in said chamber, and means for applying a voltage between said cathodes and said anode.

References Cited UNITED STATES PATENTS 3,091,717 5/1963 Rutherford et al. 230-69 X 3,112,863 12/1963 Brubaker et al. 230-69 3,161,802 12/1964 Jepsen et al. 230-69 X 3,198,422 8/1965 Kienel 230-69 ROBERT M. WALKER, Primary Examiner.

Dedication 3,398,879.-B1ian David James, Menlo Park, and Tlwodm'e K. Tom, Sunnyvale, Calif. ASYMMETRIC ION PUMP AND METHOD. Patent dated Aug. 27, 1968. Dedication filed. June 17, 1977, by the assignee, T he Perkin-E'Zmwr Uo'r'pomtio'm Hereby dedicates to the Public the entire remaining term of said patent.

[Oficial Gazette August 23, 1.977.]

Claims (1)

1. IN AN ELECTRONIC VACUUM PUMP, MEANS PROVIDING A PUMP CHAMBER, AN ANODE, TWO CATHODES EACH INCLUDING REACTIVE MATERIAL ASSOCIATED WITH OPPOSITE SIDES OF SAID ANODE, SAID ANODE AND CATHODES DISPOSED IN SAID CHAMBER, MEANS FOR APPLYING A VOLTAGE BETWEEN SAID CATHODES AND SAID ANODE, AND MEANS FOR PRODUCING A SPUTTERING YIELD FROM ONE CATHODE WHICH IS DIFFERENT THAN THE SPUTTERING YIELD FROM THE OTHER CATHODE.

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